Presently, as a next-generation communication standard of LTE (Long Term Evolution) systems, specifications of LTE-Advanced are being developed. In LTE-Advanced systems, carrier aggregation (CA) technique is introduced to achieve a higher throughput than that of the LTE systems while ensuring backward compatibility with the LTE systems. In the carrier aggregation, a component carrier (CC) having the maximum bandwidth of 20 MHz supported by the LTE systems is used as a basic component, and it is designed to achieve communication in a broader band by using these multiple component carriers simultaneously.
In the carrier aggregation, user equipment (UE) can use multiple carrier components simultaneously to communicate with a base station (evolved NodeB: eNB).
In the carrier aggregation, a highly reliable primary cell (PCell) to ensure connectivity to the user equipment and a secondary cell (SCell) additionally configured for the user equipment during connection to the primary cell are configured.
The primary cell is similar to a serving cell in the LTE systems and serves as a cell to ensure connectivity between the user equipment and a network. On the other hand, the secondary cell is a cell configured for the user equipment additionally to the primary cell.
In the carrier aggregation up to LTE Release 10 (Rel-10), as illustrated in the left side in FIG. 1, it is defined that user equipment uses multiple component carriers served from a single base station to conduct simultaneous communication. Meanwhile, in Rel-12, the carrier aggregation in Rel-10 is further extended, and as illustrated in the right side in FIG. 1, dual connectivity (DC) where the user equipment uses multiple component carriers served from multiple base stations to conduct the simultaneous communication is discussed. For example, if all component carriers cannot be accommodated in a single base station, it is considered that the dual connectivity can be effectively utilized to achieve a throughput nearly equal to that in Rel-10.
In the dual connectivity, in accordance with a process sequence as illustrated in FIG. 2, a master base station (MeNB) adds a secondary cell or a secondary cell group served by a secondary base station (SeNB) to the user equipment. As illustrated in FIG. 2, the master base station receives a measurement report indicative of an inter-frequency measurement result from the user equipment. Upon determining to additionally configure a secondary cell for the user equipment based on the received measurement report, the master base station transmits a SCG addition request to the secondary base station. Upon receiving the SCG addition request, the secondary base station returns a SCG addition request ACK including a configuration of the secondary base station. Upon receiving the SCG addition request ACK, the master base station transmits an RRC connection reconfiguration to the user equipment to additionally configure the secondary cell for the user equipment. Upon receiving the RRC connection reconfiguration, the user equipment reconfigures the configuration in accordance with the received RRC connection reconfiguration and returns an RRC connection reconfiguration complete indicative of completion of the reconfiguration to the master base station. Upon receiving the RRC connection reconfiguration complete, the master base station transmits a confirmation to the secondary base station to indicate that the user equipment has been reconfigured. On the other hand, upon receiving an RRC connection reconfiguration to indicate that a secondary cell should be additionally configured, the user equipment performs a random access (RA) procedure for the secondary base station to establishment uplink timing with the secondary base station.
See R2-141860 for further details, for example.